In this field is known patent application EP1998145 which describes a passive and reversible micro-sensor for counting the number of stress cycles experienced by a structure which may, for example, correspond to the number of temperature cycles, of tensile, compressive and/or bending mechanical stress cycles, generated, for example, by mobiles passing on this structure, whose size, advantageously, does not exceed 5 cm in its largest dimension, and preferably 2 cm, and having a virtually unlimited service life, which can be used in pyrotechnic safety, having no sensitivity to electromagnetic fields and which enables an error-free counting of this number of cycles or passages.
By reversible is meant a micro-sensor adapted to detect a cycle of variations in distance without deteriorating, thus adapted to then detect another cycle. By passive means are to be understood means operating without an energy source unlike the so-called active means used in the aforementioned patent applications and which use an energy source, namely a power supply. This micro-sensor comprises means for detecting and counting the cycles of variations in distance between two points or areas of a structure, these means comprising a support having first and second portions each having an anchoring area, these anchoring areas being adapted to be attached to one and the other of said two points or areas of the structure, respectively, and being constituted by studs, notches and/or bores and having dimensions smaller than those of the first and second portions, the counting means being associated with each of said first and second portions of the support. Also known is patent application FR2974410 which describes a passive and reversible micro-sensor for counting the number of stress cycles experienced by a structure and adapted to detect several different stress thresholds. Such a micro-sensor is shown in FIGS. 1a and 1b, without and with detecting means and counting means, respectively.
It comprises a support 29 with first and second L-shaped subassemblies 30, 31, arranged head-to-tail and separated mainly longitudinally along an axis OX by a space 32 and whose respective bases 33, 34 are, in part, anchoring areas of the support 29 on the structure to be monitored.
These bases 33, 34 each comprise two bores 15, and 17, 18. The axes Y1 and Y2 passing through the centers of the bores 15, 16 and 17, 18, respectively, are perpendicular to the OX axis, while the axes X1 and X2 passing through the centers of the bores 15, 17 and 16, 18, respectively, are parallel to the OX axis. In addition, these first and second longitudinal portions 41; 44 are connected to each other at their ends 37, 38 by an elastic member, in this case a material cord 35 and 36.
The second portion 41 of the first subassembly 30 comprises three bores 19 regularly distributed along the axis OX as well as three pairs of bores 20, the axis passing through the centers of a pair of bores being parallel to the axis Y1 and each one of the pairs is associated with one of the bores 19. Each bore 20 is intended to receive an axis projecting from the support and adapted to allow a prepositioning of non-return means.
This second portion 41 comprises as many substantially square-shaped depressions 42 as bores 19, each depression being centered around one of the bores 19. It also comprises three crenels 43 projecting from the side surface of the second portion 41 of the first subassembly located facing the second portion 44 of the second subassembly 31. For each one of the bores 19, the axis passing through its center and parallel to the axis Y1 is also an axis of symmetry of one of the crenels 43. Each one of these crenels comprises, in its middle portion, a bore 48. The second portion 44 of the second subassembly 31 comprises three pairs of bores 22 distributed the same as the bores 19 along the OX axis, each one of the pairs 22 being associated with one of the bores 19. Each bore 22 is intended to receive an axis projecting from the support and adapted to allow a prepositioning of driving means. In addition, the side surface of the second portion 44 of the second subassembly 31 located facing the second portion 41 of the first subassembly 30 comprises notches 45 of dimensions greater than those of the crenels 43 and intended to allow the introduction of the crenels therein. Each one of the bases 33, 34 is partially separated from the corresponding second portion of the L by two facing coaxial notches 46, 47.
The small notches 46 are not absolutely essential, they have however the following advantages:—facilitating the rotation of the 2 anchoring areas with respect to each other. Indeed, when the indicator is mounted on a structure subjected to bending, there is a rotation of the straight sections. Such an architecture, providing elasticity (compliance), thus allows to prevent the constraints from growing unnecessarily.—centering the base with respect to the corresponding second movable portion of the support,—leaving, at the bases, only the required material for withstanding the tensile or compressive stresses.
The large notches 47 allow to create the elastic members, namely material cords 35, 36 for securing the subassemblies 30, 31 to each other. FIG. 1b shows a perspective view of the support of FIG. 1a on which have been arranged detecting means and counting means. On the support 29 are arranged three assemblies 4, 5, 6 each comprising:                axes force-fitted into the bores 19, 20 and 22 and projecting from the support 29 and serving as a stop or an axis of rotation,        a toothed wheel 541, 542 or 543,—non-return means 551, 552 or 553—driving means 561, 562 or 563.In order to enable the detection of several different thresholds of deformations, the toothed wheels 541, 542 or 543 have a tooth pitch different from one wheel to the other.        
The devices according to these patents are sized according to the expected deformation and the technological production limitations of the several building blocks including, mainly, the tooth pitch of the counting wheels, the resolution being at most equal to that pitch.
Thus, for a given tooth pitch, the smaller the deformation to be detected and to be counted, the greater the size of the micro-sensor and therefore the greater its weight.
Yet, in some sectors such as the aeronautics field, the mass of the components should be as small as possible. Therefore, the size of the micro-sensors has to be as small as possible in consistency with the detection and the counting of the deformations.
Furthermore, the use of silicon for the manufacture of the counting wheels allows to obtain a very small tooth pitch, of the order of 100 μm, or even smaller. However, the manufacturing technology of silicon wheels is complex and it may be preferred, in some cases, to use a simpler technology such as that of metals. However, with metals, a tooth pitch of the order of 400 μm can be reached at best, which requires, for a same value of a detected and counted deformation, to implement supports at least four times larger than in the context of silicon counting wheels. Indeed, for very low amplitude deformations, the previous inventions have an architecture whose movable portion which serves to mesh with the counting wheel moves with an amplitude similar to that of the movement associated with the event. If the event is of a very low amplitude, the stroke of the movable portion may be insufficient to cause the meshing. This limitation is related to the ratio of the number of teeth to the diameter of the tooth.